I can related to that...twice!
My mother's collie, died of massive multiple heart attacks and it was pretty awful for my mother. The dog suffered too many times and the vet said it would be useless trying to keep him alive while he was going thru another heart attack on the table. So he was put down.
I had a seven year old Borzoi show dog, that died suddenly...blood clot in her heart. I thought she was just asleep but didnt move when called. She was spayed three days before and thought it was from surgery and being an older dog. Vet did an autospy and sent results to U of I and sure enough, blood clot in one of the aorta. It was sudden and she would never survived it.
So sorry to hear and time will heal. At least she lived her life as the fullest with her loving family! :aww